Controlled experiments were conducted with an enhanced experimental intermediary system, CONIT (COnnector for Networked Information Transfer), to test how effective such a system could be in assisting end users in online searching of medical and biomedical literature. A total of 16 end users, none of whom had previously operated CONIT or any of the four bibliographic retrieval systems used in the study, performed searches on 20 different topics with no assistance other than that provided by CONIT itself (except to recover from machine and software problems). The same topics were then searched by human expert intermediaries (librarians) with the end users present. Sometimes CONIT and sometimes the human expert were clearly superior in terms of such parameters as recall and search time. In general, however, end users searching alone with CONIT achieved somewhat higher online recall at the expense of longer session times. It was concluded that advanced experimental intermediary techniques are capable of providing search assistance whose effectiveness at least approximates that of human intermediaries in some contexts. Details of the enhanced CONIT system and its costs are also discussed, as well as the possibilities for even more advanced intermediary systems, including those which perform automatic database selection and simulate human experts. A 60-item bibliography is provided. (Author/ESR)